This invention relates to the use of polymer compositions based on selected alkyl (meth)acrylate monomers combined in certain weight ratios as additives to phosphate ester-based functional fluids for providing viscosity index improvement and low temperature performance in aircraft hydraulic fluids. The polymer additives are normally dissolved or dispersed in the phosphate ester-based fluids for eventual incorporation into aircraft hydraulic fluid compositions.
Functional fluids have found use as electronic coolants, diffusion pump fluids, damping fluids, heat transfer fluids, heat pump fluids, refrigeration fluids power transmission and hydraulic fluids. Hydraulic fluids intended for use in the hydraulic systems of aircraft, for example, for the operation of various mechanisms and control systems, must satisfy a variety of performance requirements. Among these requirements are good thermal stability, fire-resistance, low susceptibility to viscosity changes over a wide range of temperatures, and good fluidity at low temperatures. Viscosity index (or VI) is a measure of the degree of viscosity change as a function of temperature; high viscosity index values indicate a smaller change in viscosity with temperature variation compared to low viscosity index values. Viscosity index improver additives having high viscosity index values coupled with good low temperature fluidity allow the hyraulic fluid to flow at the lowest possible temperature of operation, such as at high altitude flight conditions, while providing satisfactory viscosity performance at higher operating temperatures.
Polymeric additives have been used to improve the performance of automobile engine lubricating oils in regard to high and low temperature viscosity characteristics. However, the functional fluids required for use in aircraft hydraulic systems are compositionally different from conventional automobile lubricating oils, such that the polymeric additives suitable for automobile engine lubricating oils are not satisfactory for use in the aircraft fluids. For example, phosphate ester fluids are of interest for use in aircraft systems because of their fire-resistant properties; however, lack of solubility in these phosphate ester-based fluids precludes the use of conventional automobile engine VI improving additives in aircraft hydraulic fluids.
U.S. Pat. No. 3,718,596 discloses the use of a mixture of high (15,000 to 40,000) and low (2,500 to 12,000) molecular weight alkyl (meth)acrylate polymers as VI improving additives in phosphate ester-based fluids to provide resistance to erosion of mechanical parts exposed to the phosphate ester fluids. Poly(butyl methacrylate) and poly(hexyl methacrylate) polymers were disclosed as high and low molecular weight polymers, respectively, for use as VI improving additives.
U.S. Pat. No. 5,464,551 discloses aircraft hydraulic fluid compositions having improved thermal, hydrolytic and oxidative stability characteristics where the phosphate ester-based compositions contain different additives that function as acid scavenger, anti-erosion agent, viscosity index improver and antioxidant. Suitable VI improving additives disclosed were poly(alkyl methacrylates) of the type described in U.S. Pat. No. 3,718,596, but with higher molecular weights (50,000 to 100,000 number average molecular weight), and where the repeating units of the poly(alkyl methacrylate) substantially comprise butyl and hexyl methacrylate.
Poly(butyl methacrylate) and poly(butyl methacrylate/dodecyl-pentadecyl methacrylate//67/33) compositions are commercially available VI improving additives prepared by conventional solution polymerization processes.
None of these previous approaches combines good viscosity index, compatibility with the phosphate ester fluids, good high temperature thickening ability at low usage levels and low temperature fluidity in a single polymer additive; it is an object of the present invention to provide this combination of properties in a single polymer additive.